The dual energy x-ray CT scan data are acquired at two energy levels. For example, the tube is set at the low and high energy levels of 80 kV and 120 kV. To use the dual energy data for material separation, the projection data undergo preconstruction decomposition. Alvarez and Macovski (1976) developed a mathematical scheme, called dual-energy decomposition, to use the dual energy information.
The physical basis of dual energy imaging includes two main mechanisms of the interaction of X rays with matter in the clinically relevant diagnostic energy-range from 30 keV to 140 keV, and the two interactions are photoelectric absorption and Compton scattering, each having its own functional dependence on X-ray energy. Photoelectric absorption is a rapidly decreasing function of energy while Compton scatter is a gentle function of energy. The photoelectric interaction is a strong function of the effective atomic number (Z) of the absorbing tissue while scattering is nearly independent of Z.
In addition to the energy dependence, dual-energy decomposition must take X-ray sources into account. Since commercial clinical CT-scanners generally use polychromatic sources, the mathematics of dual energy imaging is not trivial. In this regard, single-energy imaging with a polychromatic source does not have an exact and analytic solution. One mathematical approach in dual-energy decomposition using a polychromatic source has been described in a related U.S. application Ser. No. 12/361,280 filed on Jan. 28, 2009 and Ser. No. 12/106,907 filed on Apr. 21, 2008 as well as in a reference entitled as “Analysis of Fast kV-switching in Dual Energy CT using a Pre-reconstruction Decomposition Technique,” by Yu Zou and Michael D. Silver (2008). In dual energy computed tomography (CT), fast kV-switching techniques generally alternate voltages between projections (also called views) so that the odd (or even) projections correspond to the low (or high) tube voltage. These references are incorporated into the current application by external reference to supplement the specification. Instead of the polynomial approximation method, in the previously proposed approach combining a linear term with a non-linear beam hardening term, an iterative solution to the dual energy data domain decomposition converges rapidly due to the dominant linear term.
In the past two years, prior art attempts have implemented certain dual energy CT systems. For example, Siemens has installed a number of dual source CT-scanners, which is equipped with two X-ray sources, and each runs at a different energy level for generating the two data sets. Another example is that Philips at their Haifa research facility has developed a sandwich detector where the upper layer records the low energy data while the lower layer records the high energy data. A prototype system is installed at the Hadassah Jerusalem Hospital. In this regard, GE has developed a specialized detector using garnet for capture 2496 total projections per rotation (TPPR) at a high speed. The fast detector has been combined with a fast kV-switching X-ray source to acquire the low and high energy data sets.
Regardless of the above described dual energy data acquisition techniques, polychromatic images are reconstructed at each energy in the preconstruction decomposition approach, and each material is characterized by its “dual energy index” as described by N. J. Pelc in “Dual Energy: technical curiosity or potential clinical tool, RSNA 2007.” In further detail, the dual energy index I is expressed in the following formula.
  I  =                    HU        LOW            -              HU        HIGH                            HU        LOW            +              HU        HIGH            +      2000      where HU stands for Hunsfield Unit that is a unit of absorption and is determined by a detector.
Referring to FIG. 1, U.S. Patent Publication 2007/0092127 discloses that the two HU values (HU E1, E2) in the image space are plotted as a single data point in a feature space. For example, two attenuation values 3, 4 are detected in response to the two different energies E1, E2 of the X radiation at a detector unit, and these HU values are assigned to a corresponding pixel 2 in the X-ray image or image space 1. By the same token, pixels surrounding the pixel 2 in a predetermined area 10 each have a pair of the HU values. The HU data of these pixels are mapped into the feature space 5 according to the disclosure of U.S. Patent Publication 2007/0092127. According to the original drawing, although both X and Y axes of the feature space 5 are labeled as “HU E1,” it is assumed that one of the axes should be labeled as “HU E2” as shown in FIG. 1. Since the above example does not disclose any more details, the inventor of the current application assumes the mapping technique of U.S. Patent Publication 2007/0092127 to be performed in the following manner as illustrated in FIGS. 2A and 2B, which are not in the prior art publication.
The X and Y axes of the image space as illustrated in FIG. 2A respectively correspond to the location of pixels or detector units, and each square represents a pixel having a pair of the low and high HU values. The X and Y axes of the feature space as illustrated in FIG. 2B respectively correspond to the HU values of the low and high HU values. In this prior art example, nine pixels in a local area 10 in the image space 1 of FIG. 2A are mapped into the corresponding nine data points in the feature space 5 of FIG. 2B as indicated by the respective arrows. For example, the pixel 2 in the image space 1 has a pair of the HU values (3, 4) and is represented in the data point 2′ in the feature space 5.
Patent Publication 2007/0092127 further discloses other steps of the prior art technique to separate a certain material based upon the above described image space and feature space. To determine whether or not a give pixel 2 belongs to a particular material, the feature space in FIG. 2B has a known pre-defined region boundary 6 for the particular material. According to the prior art technique, a pixel in the local area 10 is counted if they are mapped inside the known region 6. In this example as illustrated in FIGS. 2A and 2B, among the nine pixels in the local area 10, six pixels are mapped inside the region boundary 6. Thus, the prior art technique specifically counts a number of pixels in the image domain if those pixels belong to the local area 10 surrounding the pixel 2 and are mapped inside the region 6 of the feature space 5.
According to Patent Publication 2007/0092127, the number of the pixels inside the know region is a value of an access variable, and the value is further compared to a predetermined threshold value to ultimately determine whether or not the pixel 2 belongs to the know material. In this regard, assuming the threshold value of four, since the exemplary value of six is counted for the access variable and is above the threshold value, the pixel 2 belongs to the know material. On the other hand, if the access variable value is below the threshold value, it can be further determined based a probability function whether or not the pixel belongs to a certain group of pixels. These other groups represent another material, either of the two materials and neither of the two materials. The prior art technique repeats the above described process for each pixel in the image space in order to separate a known material.
The above described prior art technique requires at least three limitations. First, the known regions in the feature space must be established in advance for each material to be separated. Secondly, the number of predetermined areas quickly becomes large. As illustrated in FIG. 1 of the prior art technique, for two know materials, a combination of six regions must be established in advance. Thirdly, even if the region boundary is established for a particular material, the region boundary may not be applicable to a given dual energy data set unless the data set is acquired under the same conditions for the established region. These conditions include an X ray source, a dosage level, a material thickness and so on. For these and other reasons, the above described prior art technique remain desired.